1. Field of the Invention
The present invention relates to spread spectrum communication receivers, and more particularly to a spread spectrum communication receiver which is suitable for mobile communications and is capable of demodulating received signals which have been distorted during propagations of multiple paths.
2. Description of the Related Art
Recently, there has been considerable activity in the research and development of a communication system using a spread spectrum communication system and a CDMA (Code Division Multiple Access) communication system which is a multiple access system utilizing the spread spectrum communication system. These communication systems are not liable to be affected by interference, disturbance and multipath propagations. A so-called RAKE receiver which combines multipath transmissions can be used in the above system, so that diversity can be obtained. Hence, the above systems are attractive as applications to mobile communications.
A spread spectrum communication receiver including a RAKE receiver will be described.
FIG. 1 is a block diagram of such a conventional spread spectrum communication receiver. The receiver includes demodulator units 1, 2 and 3, a symbol combiner 5, a transmission path state estimating unit 6, and a decision unit 7. The demodulator units 1, 2 and 3 perform respective demodulation processes on a received signal 11 in mutually independent phases. The symbol combiner 5 pulls the demodulated signals from the demodulation units 1, 2 and 3 in phase, and them combines the demodulated signals. The transmission path state estimating unit 6 estimates the states of multiple transmission paths (main lobes), and supplies the demodulator units 1, 2 and 3 with phase information indicative of the phases of the multiple paths obtained by the estimating unit 6.
FIG. 2A shows demodulation timings of the demodulator units 1 through 3. FIG. 2A shows transmission (propagation) paths (main lobes) 13, 14 and 15 and a correlation therebetween. The receive power levels of the signals respectively propagated through the paths 13, 14 and 15 have respective delay times.
The operation of the spread spectrum communication receiver shown in FIG. 1 will be described below.
The transmission path state estimating unit 6 estimates the states of the transmission paths from the received signal 11. More particularly, the estimating unit 6 measures the correlation levels in the phases of spreading codes respectively used in the demodulator units 1, 2 and 3, and thus obtains the receive power levels of the signals propagated through the paths 13, 14 and 15. The information concerning the phases and receive power levels is applied to the demodulator units 1, 2 and 3 as information 12.
The demodulator units 1 through 3 perform the demodulation processes in synchronism with the phases of the paths 13, 14 and 15 estimated by the estimating unit 6. The receiver shown in FIG. 1 has three demodulator units 1, 2 and 3, which thus perform the demodulation processes on the received signals in synchronism with the phases of the paths 13, 14 and 15, respectively. The demodulated signals output by the demodulator units 1, 2 and 3 are combined together in a maximum ratio fashion. A decision on the resultant combined signal is made by the decision unit 7, which then outputs the demodulated data.
The transmission path state estimating unit 6 provides the demodulator units 2 and 3 with relative phase difference information concerning the paths 14 and 15 with respect to the path 13. Hence, the demodulation processes of the demodulators 2 and 3 can be pulled in phase with the signals propagated through the paths 14 and 15.
However, the above conventional spread spectrum communication receiver has the following disadvantages, which will be encountered when there is a signal which is propagated through a path having a delay time longer than the symbol interval and which overlaps with a signal of the next symbol propagated through a path having a relatively small delay time.
FIG. 2B shows a path 16 located in the next symbol interval. The path 16 has a delay time longer than the symbol interval. The path 16 shown in FIG. 2B does not overlap with any path located in the next symbol interval. In this case, the spread spectrum communication receiver can demodulate the symbol from the signals propagated through the paths 13, 14 and 15 and the symbol from the signals propagated through paths 17, 18 and 19.
FIG. 2C shows that the path 16 overlaps with the path 18 in the next symbol interval. The conventional receiver does not discriminate the signals respectively propagated through the paths 16 and 18 from each other. Hence, the signal propagated through the path 16 functions as noise when the signal propagated through the path 18 is demodulated in the next symbol interval. This degrades the reception performance of the receiver.